1. Field of the Invention
The present invention relates to a method and apparatus for protecting an arrangement which is located in an area of highly-concentrated radiation, in particular, a radiation receiver of a solar tower project.
2. Discussion of the Prior Art
Due to the general shortage of energy, considerable efforts are being expended towards rendering solar energy usable. For example, it is known to direct sunbeams onto a heliostatic area (mirror panel) which reflects the incident rays and conducts them concentrated onto an effective area of, for instance, a heat exchanger system which absorbs the focused, concentrated energy in the form of heat and transfers it to another medium such as, for example, air. The hot gas which is produced in this manner is mostly employed for the operation of a hot-gas turbine for the purpose of generating electrical energy. The heliostatic area consists of a plurality of adjoiningly positioned, mutually independently controllable individual mirrors which, computer controlled, can be set to track the changing solar radiation during the course of a day in such a manner that the reflected rays will always collimatedly impinge against the effective surface. A heliostatic field can evidence a mirror diameter of up to 1000 m, wherein the effective area is located in a central, elevated position in essence, in the shape of a gas tower (up to 200 m high), which supports at its upper end the so called receiver incorporating the effective surface, (for example, a GAST receiver). The receiver is so conceived that it can be loaded up to the limit of the thermal capacity of its material. It is, therefore, extremely susceptible even to only short-term malfunctions which will lead to an overload. When the concentrated reflected field of rays is directed onto an area located outside the effective field, there is likely to ensue overheating of the outer walls of the receiver and of the gas tower when there are malfunctions in the heliostatic field or upon movement of the focal point out of the effective surface. Malfunctions in the heliostatic field may be the formation of so called "hot spots" or the failure of the (computer controlled) heliostatic control system. The mentioned overheating can also be caused by malfunctions in the heat exchanger circuit (interruption of cooling as a result of leakage, emergency shutdown of the turbine resulting from turbine disruptions).
The heliostatic field consists of individual heliostats which are capable of rapid changes about the axis of elevation and of slow changes about the axis of rotation, controlled by a central computer. The maximum speed of travel of known heliostats is approximately 320.degree./hr. Therefore it requires about two minutes to move the heliostats panel from the effective surface, the so called aperture of the receiver. Considering that a fast "emergency shut-down" (for instance, mechanical reversal of the mirrors) is not provided for in known systems due to technical reasons, receiver and gas tower regions will be overheated and frequently destroyed at full solar radiation even at known movement of the mirrors out of the aperture.
Thus, during operation, every solar tower project is subject to an unforseeable disruption (for instance, power failure of the computer control system, emergency shutdown of the turbine (about 10 secs.), as well as at a known (slow) controlling out of the heliostat mirrors from the effective field, due to the possible danger of a detrimental overheating.